The invention relates to a radiation-emitting structural element with a multilayer structure.
Radiation-emitting structural elements in the prior art have a multi-layer structure with an active layer serving to generate radiation, as well as a radiation-permeable window with a main surface, on which the multi-layer structure is placed. A window in the form of a cube or cuboid is often used in conventional structural elements.
In this form of window, a comparatively large portion of the generated radiation is totally reflected at the interface between the window and the environment, thereby reducing the decoupling of the generated radiation.
In an improved form, the window is bordered, by at least one lateral surface, which has a partial surface set at an angle to the main surface. A corresponding shape of window is schematically depicted in longitudinal section in FIG. 7.
In the structural element depicted, a multi-layer structure 3 is placed on a main surface 2 of a window 1, which structure comprises a radiation-generating active layer 4. The radiation generated in the active layer 4 and emitted in the direction of the window 1 is depicted, for illustrative purposes, by rays 5a, 5b, and 5c. These rays enter the window 1 through the main surface 2, pass through part of the window 1, and finally reach the peripheral surface of the window 1. Depending on the angle of incidence to the peripheral surface, a ray is decoupled as in the case of ray 5b, or totally reflected as in the case of 5a, 5c. Total reflection occurs when the angle of incidence, relative to the planar normal of the peripheral surface, is greater than the total angle of reflection.
In the structural element depicted, the window is bordered by lateral surfaces 7, each of which has a partial surface 9 proceeding at an angle relative to the main surface 2. As a result of the angled position, the angle of incidence of the ray 5b striking the partial surface 9 is decreased, thus reducing the proportion of totally reflected radiation and significantly increasing the overall radiation yield.
The task of the invention is to create a radiation-emitting structural element of the type described initially but with improved radiation yield. In particular, the task of the invention is to specify a structural element having high decoupling efficiency. Advantageous embodiments of the invention are the object of the dependent claims.
The idea of the invention is to limit, in a structural element of the prior art, the generation of radiation to areas in which especially high radiation decoupling occurs.
Such spatially limited generation of radiation can be accomplished by providing the active layer with high radiation decoupling in the specified areas only. In addition, the active layer can also extend across larger areas, so that, by additional means, such as a spatially limited impression of operating current, radiation is generated only in portions of the active layer. The radiation-emitting surfaces of the active layer that are in operation are referred to below as xe2x80x9cradiation-emitting surfaces.xe2x80x9d In contrast, the active layer is the layer formed within the multi-layer structure, which is fundamentally suitable for generation of radiation.
Provision is made according to the invention for formation of a radiation-emitting structural element with a multi-layer structure that has an active layer with a radiation-generating surface and a radiation-permeable window with a main surface, on which the multi-layer structure is placed. The window is bordered by a lateral surface with a first partial surface arranged perpendicular to the main surface, and a second, curved or stepped partial surface proceeding at an angle to the main surface, wherein the first partial surface merges into the second partial surface at a distance d from the main surface. The radiation-generating surface has a lateral border created, at a distance 1, from a first partial surface perpendicular to the main surface and from the edge formed by the main surface, to which the following applies:
1xe2x89xa7d/tan xcex2, where xcex2=arccos n1/n2, 
where n1 refers to the refractive index of the multi-layer structure and n2 to the refractive index of the window, which is larger than the refractive index n1 of the multi-layer structure.
As a result of this spacing of the radiation-emitting surface from the corresponding edge of the window, the portion of the radiation that strikes the first partial surface, which is perpendicular to the main surface, is decreased relative to the portion of the radiation that strikes the second partial surface, which is perpendicular to the main surface, thereby increasing radiation decoupling.
In a preferred embodiment of the invention, a third partial surface, set perpendicular to the main surface, is joined to the second, stepped or curved partial surface, which proceeds at an angle to the main surface. As a result, a base with lateral surfaces orthogonal to one another can be formed on the side of the structural element facing away from the multi-layer structure, which base is advantageous in terms of assembly of the structural element. Many existing assembly machines, especially automatic, are designed for such window shapes with orthogonal lateral surfaces and, advantageously, can also be used to some extent with this embodiment of the invention.
The radiation-generating surface is preferably recessed into an area that overlaps the window base perpendicular to the main surface, so that no radiation is generated in this area. Radiation that would be generated here would essentially be emitted in the direction of the window base, where it would be decoupled to a significantly lesser extent than at the second, angled partial surface of the lateral surface.
In an advantageous embodiment of the invention, the window has a lateral profile in the shape of a rectangle, a square, or a triangle. Here, a lateral profile is understood to mean a profile with a section plane parallel to the main surface.
More preferably, the window is bordered by a second main surface opposite and parallel to the first main surface. This configuration is advantageous, especially when the window is manufactured from a larger, planar substrate by means of sawing or breaking.
In the invention, the radiation-generating surface can comprise a plurality of radiation partial surfaces. The radiation partial surfaces are preferably arranged in the areas of the active layer in which high decoupling occurs, with the areas between the individual radiation partial surfaces reserved for low decoupling. The radiation partial surfaces are preferably framed by two or more of the aforementioned borders, and are thus characterized by especially high decoupling of the generated radiation.
In an advantageous embodiment of the invention, the window has two opposing partial surfaces, each having a first partial surface perpendicular to the main surface, with each of these first partial surfaces merging into a second, stepped or curved partial surface proceeding at an angle to the main surface. As a result of this plurality of angled partial surfaces, decoupling is advantageously increased even further. In addition, the window can also be laterally bordered on all sides by lateral surfaces with a first partial surface perpendicular to the main surface, and a second partial surface proceeding at an angle to the main surface.
Preferably, an electrical contact surface is installed on the multi-layer structure. A corresponding opposing contact surface can, for example, be installed on the side of the window facing away from the multi-layer structure. These contact surfaces serve to provide electric power to the structural element.
To form a limited radiation-emitting surface in the active layer, the contact surface installed on the multi-layer structure can be structured to correspond to the shape of the radiation-emitting surface. When current flows essentially perpendicular to the main surface, a current is introduced into those areas of the active layer that are concealed from view by the contact surface. As a result, radiation is generated only in the areas of the active layer through which current flows. Thus, by means of an electric current, the contact surface is so to speak projected onto the active layer.
In this case, the active layer can be advantageously formed to be laterally homogeneous. This facilitates production of the multi-layer structure, as structuring of the active layer is not necessary. The structuring of a contact surface generally requires little effort. Thus, the contact surface can, for example, be initially applied in a laterally homogeneous manner by vacuum metallization, and then can be structured by etching or sputtering.
Alternatively, or in addition, the multi-layer structure can have lateral peripheral surfaces that are essentially perpendicular to the main surface and simultaneously form the border of the radiation-generating surface. The structuring of the multi-layer structure required for this purpose can be accomplished by etching, for example.
In an advantageous further enhancement of the invention, the multi-layer structure comprises a radiation-generating pn-transition, which is composed of at least one p-conducting and at least one n-conducting layer. Thus, the structural element can be formed as a luminescence diode, such as an LED. In addition to being a transition formed by direct contact between the p-conducting and the n-conducting layer, a pn-transition can also refer to a transition in which the p-conducting layer is not directly adjacent to the n-conducting layer, as is the case, for example, in quantum pot structures.
In this process, the border of the radiation-emitting surface can be established by removing the p-conducting or the n-conducting layer from those areas in which no radiation is to be generated, so that no radiation-generating pn-transition is formed there. This removal can be accomplished by etching, for example.
Another method of delimitation consists in compensating p-conductivity or n-conductivity from area to area, so that a pn-transition does not also exist in those areas of the active layer in which no radiation is to be generated. Compensation of conductivity is achieved, for example, by insertion of particles of the opposing mode of conductivity.
In the invention, the multi-layer structure preferably contains GaN-based semiconductor connections. These are defined, in particular, as GaN, AlGaN, InGaN, and AlInGaN. Such connections are characterized by a high level of quantum efficiency and facilitate, particularly as a result of their comparatively large energy gap, the generation of radiation in the green, blue and ultraviolet spectral range.
A GaN-based multi-layer structure is preferably manufactured epitaxially. SiC substrates or GaN substrates, for example, are suitable as epitaxial substrates. Sapphire substrates can also be used. In the invention, the window can advantageously be manufactured from the epitaxial substrate. In this case, an SiC window is characterized by its electrical conductivity, which facilitates the formation of a vertically conductive structural element, as well as by its radiation-permeability for the generated radiation. Vertically conductive structural elements are comparatively easy to bond, and permit homogeneous current distribution in the structural element.